Multijunction metamorphic solar cells

ABSTRACT

A multijunction solar cell including interconnected first and second discrete semiconductor regions disposed adjacent and parallel to each other including first top solar subcell, second (and possibly third) lattice matched middle solar subcells; a graded interlayer adjacent to the last middle solar subcell; and a bottom solar subcell adjacent to said graded interlayer being lattice mismatched with respect to the last middle solar subcell; wherein an opening is provided from the bottom side of the semiconductor substrate to one or more of the solar subcells so as to allow a discrete electrical connector to be made extending in free space and to electrically connect contact pads on one or more of the solar subcells.

REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.16/196,578 filed Nov. 20, 2018, which is a division of U.S. patentapplication Ser. No. 15/250,643 filed Aug. 29, 2016, now U.S. Pat. No.10,270,000.

The Ser. No. 15/250,643 application claims the benefit of U.S.Provisional Application No. 62/288,181 filed Jan. 28, 2016, and U.S.Provisional Patent Application Ser. No. 62/243,239 filed Oct. 19, 2015.

This application is related to U.S. patent application Ser. Nos.14/828,197 and 14/828,206 filed Aug. 17, 2015; Ser. No. 15/210,532 filedJul. 14, 2016; and Ser. No. 15/213,594 filed Jul. 19, 2016, now U.S.Pat. No. 10,361,330, and U.S. patent application Ser. No. 15/250,673,filed Aug. 29, 2016, now U.S. Pat. No. 10,403,778.

This application is also related to U.S. patent application Ser. No.14/660,092 filed Mar. 17, 2015, which is a division of U.S. patentapplication Ser. No. 12/716,814 filed Mar. 3, 2010, now U.S. Pat. No.9,018,521; which was a continuation in part of U.S. patent applicationSer. No. 12/337,043 filed Dec. 17, 2008.

This application is also related to U.S. patent application Ser. No.13/872,663 filed Apr. 29, 2012, which was also a continuation-in-part ofapplication Ser. No. 12/337,043, filed Dec. 17, 2008.

All of the above related applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to solar cells and the fabrication ofsolar cells, and more particularly the design and specification of amultijunction solar cell using electrically coupled but spatiallyseparated semiconductor regions in a semiconductor body based on III-Vsemiconductor compounds.

Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has beenpredominantly provided by silicon semiconductor technology. In the pastseveral years, however, high-volume manufacturing of III-V compoundsemiconductor multijunction solar cells for space applications hasaccelerated the development of such technology not only for use in spacebut also for terrestrial solar power applications. Compared to silicon,III-V compound semiconductor multijunction devices have greater energyconversion efficiencies and generally more radiation resistance,although they tend to be more complex to properly specify andmanufacture. Typical commercial III-V compound semiconductormultijunction solar cells have energy efficiencies that exceed 27% underone sun, air mass 0 (AM0) illumination, whereas even the most efficientsilicon technologies generally reach only about 18% efficiency undercomparable conditions. The higher conversion efficiency of III-Vcompound semiconductor solar cells compared to silicon solar cells is inpart based on the ability to achieve spectral splitting of the incidentradiation through the use of a plurality of photovoltaic regions withdifferent band gap energies, and accumulating the current from each ofthe regions.

In satellite and other space related applications, the size, mass andcost of a satellite power system are dependent on the power and energyconversion efficiency of the solar cells used. Putting it another way,the size of the payload and the availability of on-board services areproportional to the amount of power provided. Thus, as payloads becomemore sophisticated, and applications anticipated for five, ten, twentyor more years, the power-to-weight ratio and lifetime efficiency of asolar cell becomes increasingly more important, and there is increasinginterest not only the amount of power provided at initial deployment,but over the entire service life of the satellite system, or in terms ofa design specification, the amount of power provided at the “end oflife” (EOL).

The efficiency of energy conversion, which converts solar energy (orphotons) to electrical energy, depends on various factors such as thedesign of solar cell structures, the choice of semiconductor materials,and the thickness of each subcell. In short, the energy conversionefficiency for each solar cell is dependent on the optimum utilizationof the available sunlight across the solar spectrum as well as the “age”of the solar cell, i.e. the length of time it has been deployed andsubject to degradation associated with the temperature and radiation inthe deployed space environment. As such, the characteristic of sunlightabsorption in semiconductor material, also known as photovoltaicproperties, is critical to determine the most efficient semiconductor toachieve the optimum energy conversion to meet customer requirements ofintended orbit and lifetime.

Typical III-V compound semiconductor solar cells are fabricated on asemiconductor wafer in vertical, multijunction structures or stackedsequence of solar subcells, each subcell formed with appropriatesemiconductor layers and including a p-n photoactive junction. Eachsubcell is designed to convert photons over different spectral orwavelength bands to electrical current. After the sunlight impinges onthe front of the solar cell, and photons pass through the subcells, thephotons in a wavelength band that are not absorbed and converted toelectrical energy in the region of one subcell propagate to the nextsubcell, where such photons are intended to be captured and converted toelectrical energy, assuming the downstream subcell is designed for thephoton's particular wavelength or energy band.

The individual solar cells or wafers are then disposed in horizontalarrays or panels, with the individual solar cells connected together inan electrical series and/or parallel circuit. The shape and structure ofan array, as well as the number of cells it contains, are determined inpart by the desired output voltage and current.

The energy conversion efficiency of multijunction solar cells isaffected by such factors as the number of subcells, the thickness ofeach subcell, the composition and doping of each active layer in asubcell, and the consequential band structure, electron energy levels,conduction, and absorption of each subcell, as well as its exposure toradiation in the ambient environment over time. Factors such as theshort circuit current density (J_(sc)), the open circuit voltage(V_(oc)), and the fill factor are thereby affected and are alsoimportant. Another parameter of consideration taught by the presentdisclosure is the difference between the band gap and the open circuitvoltage, or (E_(g)/q−V_(oc)), of a particular active layer, and suchparameters may vary depending on subcell layer thicknesses, doping, thecomposition of adjacent layers (such as tunnel diodes), and even thespecific wafer being examined from a set of wafers processed on a singlesupporting platter in a reactor run. Such factors also over time (i.e.during the operational life of the system). Accordingly, such parametersare NOT simple “result effective” variables (as discussed and emphasizedbelow) to those skilled in the art confronted with complex designspecifications and practical operational considerations.

One of the important mechanical or structural considerations in thechoice of semiconductor layers for a solar cell is the desirability ofthe adjacent layers of semiconductor materials in the solar cell, i.e.each layer of crystalline semiconductor material that is deposited andgrown to form a solar subcell, have similar crystal lattice constants orparameters. The present application is directed to solar cells withseveral substantially lattice matched subcells, but including at leastone subcell which is lattice mismatched, and in a particular embodimentto a five junction (5J) solar cell using electrically coupled butspatially separated four junction (4J) semiconductor devices in a singlesemiconductor body.

SUMMARY OF THE DISCLOSURE Objects of the Disclosure

It is an object of the present disclosure to provide increasedphotoconversion efficiency in a multijunction solar cell for spaceapplications over the operational life of the photovoltaic power system.

It is another object of the present disclosure to provide in amultijunction solar cell in which the selection of the composition ofthe subcells and their band gaps maximizes the efficiency of the solarcell at a predetermined high temperature (in the range of 40 to 100degrees Centigrade) in deployment in space at AM0 at a predeterminedtime after the initial deployment, such time being at least one year,and in the range of one to twenty-five years.

It is another object of the present disclosure to provide a fourjunction solar cell subassembly in which the average band gap of allfour cells in the subassembly is greater than 1.44 eV, and to couple thesubassembly in electrical series with at least one additional subcell inan adjacent solar cell subassembly.

It is another object of the present disclosure to provide a latticemis-matched five junction solar cell in which the bottom subcell isintentionally designed to have a short circuit current that issubstantially greater than current through the top three subcells whenmeasured at the “beginning-of-life” or time of initial deployment.

It is another object of the present disclosure to provide afive-junction (5J) solar assembly assembled from two four-junction (4J)solar cell subassemblies in an integrated semiconductor structure sothat the total current provided by the two subassemblies matches thetotal current handling capability of the bottom subcell of the assembly.

It is another object of the present disclosure to match the larger shortcircuit current of the bottom subcell of the solar cell assembly withtwo or three parallel stacks of solar subcells, i.e. a configuration inwhich the value of the short circuit current of the bottom subcell is atleast twice, or at least three times, that of the solar subcells in eachparallel stack which are connected in a series with the bottom subcell.Stated another way, given the choice of the composition of the bottomsubcell, and there by the short circuit current of the bottom subcell,it is an object of the disclosure that the upper subcell stack bespecified and designed to have a short circuit which is one-half or lessthan that of the bottom subcell.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingobjects.

Features of the Invention

The present application is directed to solar cells with severalsubstantially lattice matched subcells, but in some embodimentsincluding at least one subcell which is lattice mismatched, and in aparticular embodiment to a five junction (5J) solar cell usingelectrically coupled but spatially separated four junction (4J)semiconductor regions in a semiconductor body based on III-Vsemiconductor compounds.

All ranges of numerical parameters set forth in this disclosure are tobe understood to encompass any and all subranges or “intermediategeneralizations” subsumed herein. For example, a stated range of “1.0 to2.0 eV” for a band gap value should be considered to include any and allsubranges beginning with a minimum value of 1.0 eV or more and endingwith a maximum value of 2.0 eV or less, e.g., 1.0 to 2.0, or 1.3 to 1.4,or 1.5 to 1.9 eV.

Briefly, and in general terms, the present disclosure describes solarcells that include a solar cell assembly of two or more solar cellsubassemblies in a single monolithic semiconductor body composed of atandem stack of solar subcells, where the subassemblies areinterconnected electrically to one another.

As described in greater detail, the present application discloses thatinterconnecting two or more spatially split multijunction solar cellregions or subassemblies can be advantageous. The spatial split can beprovided for multiple solar cell subassemblies monolithically formed ona single substrate and remaining as a monolithic semiconductor body withdistinct characteristics. Alternatively, the solar cell subassembliescan be physically separated or fabricated individually as separatesemiconductor chips that can be coupled together electrically. (Suchalternative embodiments are covered in parallel applications, such asSer. No. 15/213,594, noted in the Reference to Related Applications).

One advantage of interconnecting two or more spatially splitmultijunction solar cell subassemblies is that such an arrangement canallow accumulation of the current from the upper subcells in theadjacent semiconductor bodies into the bottom subcells which have highercurrent generation ability.

One advantage of interconnecting two or more spatially splitmultijunction solar cell subassemblies is that such an arrangement canallow the bottom subcells of different subassemblies to be connected inelectrical series, this boosting the maximum operational voltage andopen circuit voltage associated with the solar cell assembly, andthereby improving efficiency.

Further, selection of relatively high band gap semiconductor materialsfor the top subcells can provide for increased photoconversionefficiency in a multijunction solar cell for outer space or otherapplications over the operational life of the photovoltaic power system.For example, increased photoconversion efficiency at a predeterminedtime (measured in terms of five, ten, fifteen or more years) afterinitial deployment of the solar cell can be achieved.

Thus, in one aspect, a monolithic solar cell subassembly includes afirst semiconductor body including an upper first solar subcell composedof (aluminum) indium gallium phosphide ((Al)InGaP); a second solarsubcell disposed adjacent to and lattice matched to said upper firstsubcell, the second solar subcell composed of (aluminum) (indium)gallium arsenide ((Al)(In)GaAs) or indium gallium arsenide phosphide(InGaAsP); and a bottom subcell lattice matched to said second subcelland composed of (indium) gallium arsenide (In)GaAs.

The aluminum (or Al) constituent element, or indium (or In), shown inparenthesis in the preceding formula means that Al or In (as the casemay be) is an optional constituent, and in the case of Al, in thisinstance may be used in an amount ranging from 0% to 40% by molefraction. In some embodiments, the amount of aluminum may be between 20%and 30%. The subcells are configured so that the current density of theupper first subcell and the second subcell have a substantially equalpredetermined first value, and the current density of the bottom subcellis at least twice that of the predetermined first value.

Briefly, and in general terms, the present disclosure provides a fivejunction solar cell comprising a semiconductor body including:

(a) a first semiconductor region including:

an upper first solar subcell composed of a semiconductor material havinga first band gap, and including a top contact on the top surfacethereof;

a second solar subcell adjacent to said first solar subcell and composedof a semiconductor material having a second band gap smaller than thefirst band gap and being lattice matched with the upper first solarsubcell;

a third solar subcell adjacent to said second solar subcell and composedof a semiconductor material having a third band gap smaller than thesecond band gap and being lattice matched with the second solar subcell;

an interlayer adjacent to said third solar subcell, said interlayerhaving a fourth band gap or band gaps greater than said third band gap;and

a fourth solar subcell adjacent to said interlayer and composed of asemiconductor material having a fifth band gap smaller than the fourthband gap and being lattice mismatched with the third solar subcell, andincluding a first contact on the top surface thereof, and a secondcontact on the bottom surface thereof;

(b) a second semiconductor region disposed adjacent and parallel to thefirst semiconductor region and including:

an upper first solar subcell composed of a semiconductor material havinga first band gap, and including a top contact on the top surfacethereof;

a second solar subcell adjacent to said first solar subcell and composedof a semiconductor material having a second band gap smaller than thefirst band gap and being lattice matched with the upper first solarsubcell;

a third solar subcell adjacent to said second solar subcell and composedof a semiconductor material having a third band gap smaller than thesecond band gap and being lattice matched with the second solar subcelland having a bottom contact;

an interlayer adjacent to said third solar subcell, said interlayerhaving a fourth band gap greater than said third band gap; and

a fourth solar subcell adjacent to said interlayer and composed of asemiconductor material having a fifth band gap smaller than the fourthband gap and being lattice mismatched with the third solar subcell, andincluding a first contact on the top surface thereof, and a secondcontact on the bottom surface thereof connected to the terminal of asecond polarity;

(c) wherein the top contact of the first semiconductor region iselectrically coupled with the top contact of the second semiconductorregion and to a terminal of first polarity;

wherein the first contact on the top surface of the fourth solar subcellof the first semiconductor region is electrically coupled with thebottom contact of the third solar subcell of the second semiconductorregion; and

the second contact on the bottom surface of the fourth solar subcell ofthe first semiconductor region is electrically coupled with the firstcontact on the top surface of the fourth solar subcell of the secondsemiconductor region thereof so as to form a five junction solar cell.

In some embodiments, the interlayer in each of the first and secondsemiconductor region is compositionally graded to substantially latticematch the upper solar subcell on one side and the adjacent lower solarsubcell on the other side, and is composed of any of the As, P, N, Sbbased III-V compound semiconductors subject to the constraints of havingthe in-plane lattice parameter less than or equal to that of the thirdsolar subcell on the first surface and greater than or equal to that ofthe lower fourth solar subcell on the other opposing surface.

In some embodiments, the interlayer in each of the first and secondsemiconductor bodies is compositionally graded to substantially latticematch the third solar subcell on one side and the lower fourth solarsubcell on the other side, and is composed of(In_(x)Ga_(1-x))Al_(1-y)As_(y), In_(x)Ga_(1-x)P, or (Al)In_(x)Ga_(1-x)Ascompound semiconductors subject to the constraints of having thein-plane lattice parameter less than or equal to that of the third solarsubcell and greater than or equal to that of the lower fourth solarsubcell.

In some embodiments, the fourth subcell has a band gap of approximately0.67 eV, the third subcell has a band gap in the range of 1.41 eV and1.31 eV, the second subcell has a band gap in the range of 1.65 to 1.8eV and the upper first subcell has a band gap in the range of 2.0 to2.20 eV.

In some embodiments, the third subcell has a band gap of approximately1.37 eV, the second subcell has a band gap of approximately 1.73 eV andthe upper first subcell has a band gap of approximately 2.10 eV.

In some embodiments, the upper first subcell is composed of indiumgallium aluminum phosphide; the second solar subcell includes an emitterlayer composed of indium gallium phosphide or aluminum gallium arsenide,and a base layer composed of aluminum gallium arsenide; the third solarsubcell is composed of indium gallium arsenide; the fourth subcell iscomposed of germanium or InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn,InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN,and the graded interlayer is composed of (Al)In_(x)Ga_(1-x)As orIn_(x)Ga_(1-x)P with 0<x<1, and (Al) denotes that aluminum is anoptional constituent.

In some embodiments, the upper first solar subcell, the second solarsubcell, the third solar subcell, and the interlayer in the first andsecond semiconductor region form an integral monolithic semiconductorbody. The semiconductor regions are “parallel” to each other in that theregions are disposed adjacent and parallel to one another so that theincoming light illuminates both the upper first solar subcell of thefirst semiconductor region and the first solar subcell of the secondsemiconductor region, and that parallel light beams traverses the stackof subcells of the entire semiconductor body.

In some embodiments, the first and second semiconductor regionconstitute a single semiconductor body that has been etched from thebackside so that the substrate is separated into two spatially separatedinterconnected regions.

In some embodiments, the band gap of the interlayer is in the range of1.41 eV to 1.6 eV throughout its thickness.

In some embodiments, the first and second semiconductor regionsconstitute a single semiconductor body that has been isolated to formtwo spatially separated and electrically interconnected solar cellsubassemblies.

In some embodiments, the respective selection of the composition, bandgaps, open circuit voltage, and short circuit current of each of thesubcells (i) maximizes the efficiency of the assembly at hightemperature (in the range of 40 to 100 degrees Centigrade) in deploymentin space at a predetermined time after the initial deployment (referredto as the “beginning of life” or BOL), such predetermined time beingreferred to as the “end-of-life” (EOL), wherein such predetermined timeis in the range of one to twenty-five years; or (ii) maximizes theefficiency of the solar cell at a predetermined low intensity (less than0.1 suns) and low temperature value (less than minus 80 degreesCentigrade) in deployment in space at a predetermined time after theinitial deployment in space, or the “beginning of life” (BOL), suchpredetermined time being referred to as the “end-of-life” (EOL) time,and being at least one year.

In some embodiments, the amount of aluminum in the upper first subcellis at least 10% by mole fraction.

In some embodiments, the semiconductor body further comprises a firsthighly doped lateral conduction layer disposed adjacent to and above thefourth solar subcell and a blocking p-n diode or insulating layerdisposed adjacent to and above the first highly doped lateral conductionlayer.

In some embodiments, the semiconductor body further comprises a secondhighly doped lateral conduction layer disposed adjacent to and above theblocking p-n diode or insulating layer.

In some embodiments, there further comprises a first alpha layerdisposed above the second lateral conduction layer and having adifferent composition and a thickness of between 0.25 and 1.0 micronsand functioning to prevent threading dislocations from propagating,either opposite to the direction of growth or in the direction of growthinto the second subcell.

In some embodiments, the short circuit current density (J_(sc)) of thefirst, second and third middle subcells are approximately 11 mA/cm², andthe short circuit current density (J_(sc)) of the bottom subcell isapproximately 34 mA/cm².

In some embodiments, the short circuit density (J_(sc)) of the bottomsubcell is at least three times that of the first, second and thirdsubcells, with the base region of such subcell having a gradation indoping that increases from the base-emitter junction to the bottom ofthe base region in the range of 1×10¹⁵ to 5×10¹⁸ per cubic centimeter.

In another aspect, the present disclosure provides a multijunction solarcell including a terminal of first polarity and a terminal of secondpolarity comprising first and second semiconductor regions in a singlesemiconductor body, each region including substantially identical tandemvertical stacks of at least an upper first and a second bottom solarsubcell in which the second semiconductor region is disposed adjacent toand with respect to the incoming illumination, parallel to the firstsemiconductor region; a bottom contact on the bottom subcell of thesecond semiconductor region connected to the terminal of secondpolarity; a top electric contact on both the upper first subcells of thefirst and second semiconductor regions electrically connected to the topelectrical contacts to the terminal of first polarity; and an electricalinterconnect connecting the bottom second subcell of the firstsemiconductor region in a series electrical circuit with the bottomsecond subcell of the second semiconductor region so that at least athree junction solar cell is formed by the electrically interconnectedsemiconductor regions.

In another aspect, the present disclosure provides a method comprising:

(a) growing a sequence of semiconductor layers on a substrate forming asemiconductor body, the sequence of layers including an upper firstsolar subcell composed of a semiconductor material having a first bandgap, and including a top contact region on the top surface thereof;

a second solar subcell adjacent to said first solar subcell and composedof a semiconductor material having a second band gap smaller than thefirst band gap and being lattice matched with the upper first solarsubcell;

a third solar subcell adjacent to said second solar subcell and composedof a semiconductor material having a third band gap smaller than thesecond band gap and being lattice matched with the second solar subcell;

a graded interlayer adjacent to said third solar subcell, said gradedinterlayer having a fourth band gap greater than said third band gap;and

a fourth solar subcell adjacent to said third solar subcell and composedof a semiconductor material having a fifth band gap smaller than thefourth band gap and being lattice mismatched with the third solarsubcell, and including a first contact on the top surface thereof, and asecond contact on the bottom surface thereof;

wherein the graded interlayer is compositionally graded to lattice matchthe third solar subcell on one side and the lower fourth solar subcellon the other side, and is composed of any of the As, P, N, Sb basedIII-V compound semiconductors subject to the constraints of having thein-plane lattice parameter less than or equal to that of the third solarsubcell and greater than or equal to that of the lower fourth solarsubcell,

(b) etching the semiconductor body from the substrate side to form firstand second adjacent but electrically isolated semiconductor regions,each region including:

the upper first solar subcell;

the second solar subcell;

the third solar subcell;

the graded interlayer; and

in each region, a distinct and spatially separated fourth solar subcell.

In some embodiments, there further comprises:

(c) forming electrical connections so that the top contact region of thefirst semiconductor region is electrically coupled with the top contactof the second semiconductor region;

the first contact region on the top surface of the fourth solar subcellof the first semiconductor region is electrically coupled with the firstcontact region on the top surface of the fourth solar subcell of thesecond semiconductor region; and the second contact region on the bottomsurface of the fourth solar subcell of the first semiconductor region iselectrically coupled with the first contact region on the top surface ofthe fourth solar subcell of the second semiconductor region thereof.

In some embodiments, the short circuit density (J_(sc)) of the bottomsubcell is at least three times that of the first, second and thirdsubcells.

In some embodiments, the semiconductor body further comprises a firstopening in the backside of the body extending from a bottom surface ofthe semiconductor body to the first lateral conduction layer; a secondopening in the semiconductor body extending from the bottom surface ofthe first semiconductor body to the second lateral conduction layer; anda third opening in the first semiconductor body extending from a surfaceof the semiconductor body to the p-type semiconductor material of thebottom subcell of the semiconductor body.

In some embodiments, the solar cell assembly further comprises a firstmetallic contact pad disposed on the first lateral conduction layer ofeach of the semiconductor regions; and a second metallic contact paddisposed on the second lateral conduction layer of the semiconductorbody; and an electrical interconnect connecting the first and secondcontact pads.

In some embodiments, the solar cell assembly further comprises a thirdmetallic contact pad disposed on the second lateral conduction layer ofthe semiconductor regions; a fourth metallic contact pad disposed on thep-type semiconductor material of the bottom subcell of the semiconductorbody; and an electrical interconnect connecting the third and fourthcontact pads.

In another aspect, the present disclosure provides a multijunction solarcell including a terminal of first polarity and a terminal of secondpolarity comprising first and second semiconductor regions includingsubstantially identical tandem vertical stacks of at least an upperfirst and a bottom second solar subcell lattice mismatched to the upperfirst solar subcell in which the second semiconductor region is disposedadjacent and parallel to the first semiconductor region; a bottomcontact on the bottom second subcell of the second semiconductor regionconnected to the terminal of second polarity; a top electric contact onboth the upper first subcells of the first and second semiconductorregion electrically connected to the top electrical contacts to theterminal of first polarity; and an electrical interconnect connectingthe bottom second subcell of the first semiconductor region in a serieselectrical circuit with the bottom second subcell of the secondsemiconductor region so that at least a multijunction solar cell isformed by the electrically interconnected semiconductor region. Theseries connection of the bottom subcell of the second semiconductorregion incrementally increases the aggregate open circuit voltage of theassembly by 0.25 volts and the operating voltage by 0.21 volts, therebyincreasing the power output of the solar cell.

In some embodiments, the average band gap of all four subcells (i.e.,the sum of the four band gaps of each subcell divided by four) in thesemiconductor body is greater than 1.44 eV, and the fourth subcell iscomprised of a direct or indirect band gap material such that the lowestdirect band gap of the material is greater than 0.75 eV.

In some embodiments, the fourth subcell is comprised of a direct orindirect band gap material such that the lowest direct band gap of thematerial is less than 0.90 eV.

In some instances, the upper first subcell of the first semiconductorbody is composed of aluminium indium gallium phosphide (AlInGaP); thesecond solar subcell of the first semiconductor body is disposedadjacent to and lattice matched to said upper first subcell, and iscomposed of aluminum indium gallium arsenide (Al(In)GaAs); and the thirdsubcell is disposed adjacent to said second subcell and is composed ofindium gallium arsenide (In)GaAs.

In some cases (e.g., for an assembly having two subassemblies), theshort circuit density (J_(sc)/cm²) of each of the first and secondsubcells is approximately 12 mA/cm². In other instances (e.g., for anassembly having three subassemblies), the short circuit density(J_(sc)/cm²) of each of the first, second and third middle subcells isapproximately 10 mA/cm². The short circuit density (J_(sc)/cm²) of thebottom subcell in the foregoing cases can be approximately greater than24 mA/cm² or greater than 30 mA/cm². However, the short circuitdensities (J_(sc)/cm²) may have different values in someimplementations.

In some embodiments, the band gap of the graded interlayer may be eitherconstant or may vary throughout the thickness of the interlayer.

In some embodiments, there further comprises a distributed Braggreflector (DBR) layer adjacent to and between the third and the fourthsolar subcells and arranged so that light can enter and pass through thethird solar subcell and at least a portion of which can be reflectedback into the third solar subcell by the DBR layer.

In some embodiments, the distributed Bragg reflector layer is composedof a plurality of alternating layers of lattice matched materials withdiscontinuities in their respective indices of refraction.

In some embodiments, the difference in refractive indices betweenalternating layers is maximized in order to minimize the number ofperiods required to achieve a given reflectivity, and the thickness andrefractive index of each period determines the stop band and itslimiting wavelength.

In some embodiments, the DBR layer includes a first DBR layer composedof a plurality of n type or p type Al_(x)Ga_(1-x)(In)As layers, and asecond DBR layer disposed over the first DBR layer and composed of aplurality of n type or p type Al_(y)Ga_(1-y)(In)As layers, where 0<x<1,0<y<1, and y is greater than x, and (In) designates that indium is anoptional constituent.

In another aspect, the present disclosure provides a multijunction solarcell and its method of manufacture including interconnected first andsecond discrete semiconductor regions disposed adjacent and parallel toeach other in a single semiconductor body, including first top subcell,second (and possibly third) lattice matched middle subcells; a gradedinterlayer adjacent to the last middle solar subcell; and a bottom solarsubcell adjacent to said graded interlayer being lattice mismatched withrespect to the last middle solar subcell; wherein the interconnectedregions form at least a four junction solar cell by a series connectionbeing formed between the bottom solar subcell in the first semiconductorregion and the bottom solar subcell in the second semiconductor region.

In another aspect, the present disclosure provides a method ofmanufacturing a five junction solar cell comprising providing agermanium substrate; growing on the germanium substrate a sequence oflayers of semiconductor material using a MOCVD semiconductor dispositionprocess to form a solar cell comprising a plurality of subcellsincluding a metamorphic layer, growing a third subcell over themetamorphic layer having a band gap of approximately 1.30 eV to 1.41 eV,growing a second subcell over the third subcell having a band gap in therange of approximately 1.65 to 1.8 eV, and growing an upper firstsubcell over the second subcell having a band gap in the range of 2.0 to2.20 eV.

In some embodiments, there further comprises (i) a back surface field(BSF) layer disposed directly adjacent to the bottom surface of thethird subcell, and (ii) at least one distributed Bragg reflector (DBR)layer directly below the BSF layer so that light can enter and passthrough the first, second and third subcells and at least a portion ofwhich be reflected back into the third subcell by the DBR layer.

In some embodiments, the fourth (i.e., bottom) subcell of each of thesolar cell subassemblies is composed of germanium. The indirect band gapof the germanium at room temperature is about 0.66 eV, while the directband gap of germanium at room temperature is 0.8 eV. Those skilled inthe art with normally refer to the “band gap” of germanium as 0.66 eV,since it is lower than the direct band gap value of 0.8 eV. Thus, insome implementations, the fourth subcell has a direct band gap ofgreater than 0.75 eV. Reference to the fourth subcell having a directband gap of greater than 0.75 eV is expressly meant to include germaniumas a possible semiconductor material for the fourth subcell, althoughother semiconductor materials can be used as well. In other instances,the fourth subcell may have a direct band gap of less than 0.90 eV. Forexample, the fourth subcell may be composed of InGaAs, GaAsSb, InAsP,InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, or other III-V orII-VI compound semiconductor materials.

In some embodiments, additional layer(s) may be added or deleted in thecell structure without departing from the scope of the presentdisclosure.

Some implementations can include additional solar subcells in one ormore of the semiconductor bodies.

Some implementations of the present disclosure may incorporate orimplement fewer of the aspects and features noted in the foregoingsummaries.

Additional aspects, advantages, and novel features of the presentdisclosure will become apparent to those skilled in the art from thisdisclosure, including the following detailed description as well as bypractice of the disclosure. While the disclosure is described below withreference to preferred embodiments, it should be understood that thedisclosure is not limited thereto. Those of ordinary skill in the arthaving access to the teachings herein will recognize additionalapplications, modifications and embodiments in other fields, which arewithin the scope of the disclosure as disclosed and claimed herein andwith respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better and more fully appreciated by reference tothe following detailed description when considered in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a graph representing the BOL value of the parameterE_(g)/q−V_(oc) at 28° C. plotted against the band gap of certain ternaryand quaternary materials defined along the x-axis;

FIG. 2A is a cross-sectional view of a first embodiment of asemiconductor body including a four solar subcells after several stagesof fabrication including the growth of certain semiconductor layers onthe growth substrate up to the contact layer, according to the presentdisclosure;

FIG. 2B is a cross-sectional view of a second embodiment of asemiconductor body including a four solar subcells including two latticemismatched subcells with a metamorphic layer between them, after severalstages of fabrication including the growth of certain semiconductorlayers on the growth substrate up to the contact layer, according to thepresent disclosure;

FIG. 2C is a cross-sectional view of the embodiment of FIG. 2B followingthe steps of etching contact ledges on various semiconductor layersaccording to a first implementation in the present disclosure;

FIG. 2D is a cross-sectional view of the embodiment of FIG. 2B followingthe steps of etching contact ledges on various semiconductor layersaccording to a second implementation in the present disclosure;

FIG. 2E is a cross-sectional view of the embodiment of FIG. 2D followingelectrical connection of the first and second semiconductor regions bydiscrete electrical interconnects according to the present disclosure;

FIG. 3 is a bottom plan view of the solar cell of FIG. 2E depicting theelectrical interconnects;

FIG. 4 is a graph of the doping profile in the base and emitter layersof a subcell in the solar cell according to the present disclosure; and

FIG. 5 is a schematic diagram of the five junction solar cell of FIG.2E.

GLOSSARY OF TERMS

“III-V compound semiconductor” refers to a compound semiconductor formedusing at least one elements from group III of the periodic table and atleast one element from group V of the periodic table. III-V compoundsemiconductors include binary, tertiary and quaternary compounds. GroupIII includes boron (B), aluminum (Al), gallium (Ga), indium (In) andthallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb) and bismuth (Bi).

“Band gap” refers to an energy difference (e.g., in electron volts (eV))separating the top of the valence band and the bottom of the conductionband of a semiconductor material.

“Beginning of Life (BOL)” refers to the time at which a photovoltaicpower system is initially deployed in operation.

“Bottom subcell” refers to the subcell in a multijunction solar cellwhich is furthest from the primary light source for the solar cell.

“Compound semiconductor” refers to a semiconductor formed using two ormore chemical elements.

“Current density” refers to the short circuit current density J_(sc)through a solar subcell through a given planar area, or volume, ofsemiconductor material constituting the solar subcell.

“Deposited”, with respect to a layer of semiconductor material, refersto a layer of material which is epitaxially grown over anothersemiconductor layer.

“End of Life (EOL)” refers to a predetermined time or times after theBeginning of Life, during which the photovoltaic power system has beendeployed and has been operational. The EOL time or times may, forexample, be specified by the customer as part of the required technicalperformance specifications of the photovoltaic power system to allow thesolar cell designer to define the solar cell subcells and sublayercompositions of the solar cell to meet the technical performancerequirement at the specified time or times, in addition to other designobjectives. The terminology “EOL” is not meant to suggest that thephotovoltaic power system is not operational or does not produce powerafter the EOL time.

“Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

“Inverted metamorphic multijunction solar cell” or “IMM solar cell”refers to a solar cell in which the subcells are deposited or grown on asubstrate in a “reverse” sequence such that the higher band gapsubcells, which would normally be the “top” subcells facing the solarradiation in the final deployment configuration, are deposited or grownon a growth substrate prior to depositing or growing the lower band gapsubcells.

“Layer” refers to a relatively planar sheet or thickness ofsemiconductor or other material. The layer may be deposited or grown,e.g., by epitaxial or other techniques.

“Lattice mismatched” refers to two adjacently disposed materials orlayers (with thicknesses of greater than 100 nm) having in-plane latticeconstants of the materials in their fully relaxed state differing fromone another by less than 0.02% in lattice constant. (Applicant expresslyadopts this definition for the purpose of this disclosure, and notesthat this definition is considerably more stringent than that proposed,for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6%lattice constant difference).

“Metamorphic layer” or “graded interlayer” refers to a layer thatachieves a gradual transition in lattice constant generally throughoutits thickness in a semiconductor structure.

“Middle subcell” refers to a subcell in a multijunction solar cell whichis neither a Top Subcell (as defined herein) nor a Bottom Subcell (asdefined herein).

“Short circuit current (I_(sc))” refers to the amount of electricalcurrent through a solar cell or solar subcell when the voltage acrossthe solar cell is zero volts, as represented and measured, for example,in units of milliamps.

“Short circuit current density”—see “current density”.

“Solar cell” refers to an electronic device operable to convert theenergy of light directly into electricity by the photovoltaic effect.

“Solar cell assembly” refers to two or more solar cell subassembliesinterconnected electrically with one another.

“Solar cell subassembly” refers to a stacked sequence of layersincluding one or more solar subcells.

“Solar subcell” refers to a stacked sequence of layers including a p-nphotoactive junction composed of semiconductor materials. A solarsubcell is designed to convert photons over different spectral orwavelength bands to electrical current.

“Substantially current matched” refers to the short circuit currentthrough adjacent solar subcells being substantially identical (i.e.within plus or minus 1%).

“Top subcell” or “upper subcell” refers to the subcell in amultijunction solar cell which is closest to the primary light sourcefor the solar cell.

“ZTJ” refers to the product designation of a commercially availableSolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of the actual embodiment nor the relative dimensions ofthe depicted elements, and are not drawn to scale.

A variety of different features of multijunction solar cells (as well asinverted metamorphic multijunction solar cells) are disclosed in therelated applications noted above. Some, many or all of such features maybe included in the structures and processes associated with thenon-inverted or “upright” solar cells of the present disclosure.However, more particularly, the present disclosure is directed to thefabrication of a multijunction lattice matched solar cell grown over ametamorphic layer which is grown on a single growth substrate whichcomprises two or more interconnected solar cell subassemblies. Morespecifically, however, in some embodiments, the present disclosurerelates to a multijunction solar cell with direct band gaps in the rangeof 2.0 to 2.15 eV (or higher) for the top subcell, and (i) 1.65 to 1.8eV, and (ii) 1.41 eV for the middle subcells, and 0.6 to 0.9 eV director indirect band gaps, for the bottom subcell(s), respectively, and theconnection of two or more such subassemblies to form a solar cellassembly.

The conventional wisdom for many years has been that in a monolithicmultijunction tandem solar cell, “ . . . the desired opticaltransparency and current conductivity between the top and bottom cells .. . would be best achieved by lattice matching the top cell material tothe bottom cell material. Mismatches in the lattice constants createdefects or dislocations in the crystal lattice where recombinationcenters can occur to cause the loss of photogenerated minority carriers,thus significantly degrading the photovoltaic quality of the device.More specifically, such effects will decrease the open circuit voltage(V_(oc)), short circuit current (J_(sc)), and fill factor (FF), whichrepresents the relationship or balance between current and voltage foreffective output” (Jerry M. Olson, U.S. Pat. No. 4,667,059, “Current andLattice Matched Tandem Solar Cell”).

As progress has been made toward higher efficiency multijunction solarcells with four or more subcells, nevertheless, “it is conventionallyassumed that substantially lattice-matched designs are desirable becausethey have proven reliability and because they use less semiconductormaterial than metamorphic solar cells, which require relatively thickbuffer layers to accommodate differences in the lattice constants of thevarious materials” (Rebecca Elizabeth Jones-Albertus et al., U.S. Pat.No. 8,962,993).

Even more recently “ . . . current output in each subcell must be thesame for optimum efficiency in the series—connected configuration”(Richard R. King et al., U.S. Pat. No. 9,099,595).

The present disclosure provides a solar cell subassembly with anunconventional four junction design (with three grown lattice matchedsubcells, which are lattice mismatched to the Ge substrate) that leadsto significant performance improvement over that of traditional threejunction solar cell on Ge despite the substantial current mismatchpresent between the top three junctions and the bottom Ge junction. Thisperformance gain is especially realized at high temperature and afterhigh exposure to space radiation by the proposal of incorporating highband gap semiconductors that are inherently more resistant to radiationand temperature.

As described in greater detail, the present application further notesthat interconnecting two or more spatially split multijunction solarcell subassemblies (with each subassembly incorporating Applicant'sunconventional design) can be even more advantageous. The spatial splitcan be provided for multiple solar cell subassemblies monolithicallyformed on the same substrate according to the present disclosure.Alternatively, the solar cell subassemblies can be fabricated asseparate semiconductor chips that can be coupled together electrically,as described in related applications.

In general terms, a solar cell assembly in accordance with one aspect ofthe present disclosure, can include a terminal of first polarity and aterminal of second polarity. The solar cell assembly includes a firstsemiconductor subassembly including a tandem vertical stack of at leasta first upper, a second, third and fourth bottom solar subcells, thefirst upper subcell having a top contact connected to the terminal offirst polarity. A second semiconductor subassembly is disposed adjacentto the first semiconductor subassembly and includes a tandem verticalstack of at least a first upper, a second, third, and fourth bottomsolar subcells, the fourth bottom subcell having a back side contactconnected to the terminal of second polarity. The fourth subcell of thefirst semiconductor subassembly is connected in a series electricalcircuit with the third subcell of the second semiconductor subassembly.Thus, a five-junction solar assembly is assembled from two four-junctionsolar cell subassemblies.

In some cases, the foregoing solar cell assembly can provide increasedphotoconversion efficiency in a multijunction solar cell for outer spaceor other applications over the operational life of the photovoltaicpower system.

Another aspect of the present disclosure is that to provide a fivejunction solar cell assembly composed of an integral semiconductor bodywith two interconnected spatially separated four junction solar cellsubassemblies or regions, the average band gap of all four subcells(i.e., the sum of the four band gaps of each subcell divided by 4) ineach solar cell subassembly being greater than 1.44 eV.

Another descriptive aspect of the present disclosure is to characterizethe fourth subcell as being composed of an indirect or direct band gapmaterial such that the lowest direct band gap is greater than 0.75 eV,in some embodiments.

Another descriptive aspect of the present disclosure is to characterizethe fourth subcell as being composed of a direct band gap material suchthat the lowest direct band gap is less than 0.90 eV, in someembodiments.

In some embodiments, the fourth subcell in each solar cell subassemblyis germanium, while in other embodiments the fourth subcell is InGaAs,GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi,InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN or other III-V or II-VIcompound semiconductor material.

The indirect band gap of germanium at room temperature is about 0.66 eV,while the direct band gap of germanium at room temperature is 0.8 eV.Those skilled in the art will normally refer to the “band gap” ofgermanium as 0.66 eV, since it is lower than the direct band gap valueof 0.8 eV.

The recitation that “the fourth subcell has a direct band gap of greaterthan 0.75 eV” is therefore expressly meant to include germanium as apossible semiconductor for the fourth subcell, although othersemiconductor materials can be used as well.

More specifically, the present disclosure intends to provide arelatively simple and reproducible technique that does not employinverted processing associated with inverted metamorphic multijunctionsolar cells, and is suitable for use in a high volume productionenvironment in which various semiconductor layers are grown on a growthsubstrate in an MOCVD reactor, and subsequent processing steps aredefined and selected to minimize any physical damage to the quality ofthe deposited layers, thereby ensuring a relatively high yield ofoperable solar cells meeting specifications at the conclusion of thefabrication processes.

The lattice constants and electrical properties of the layers in thesemiconductor structure are preferably controlled by specification ofappropriate reactor growth temperatures and times, and by use ofappropriate chemical composition and dopants. The use of a depositionmethod, such as Molecular Beam Epitaxy (MBE), Organo Metallic VaporPhase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD),or other vapor deposition methods for the growth may enable the layersin the monolithic semiconductor structure forming the cell to be grownwith the required thickness, elemental composition, dopant concentrationand grading and conductivity type.

The present disclosure is directed to, in one embodiment, a growthprocess using a metal organic chemical vapor deposition (MOCVD) processin a standard, commercially available reactor suitable for high volumeproduction. More particularly, the present disclosure is directed to thematerials and fabrication steps that are particularly suitable forproducing commercially viable multijunction solar cells usingcommercially available equipment and established high-volume fabricationprocesses, as contrasted with merely academic expositions of laboratoryor experimental results.

Prior to discussing the specific embodiments of the present disclosure,a brief discussion of some of the issues associated with the design ofmultijunction solar cells, and in particular metamorphic solar cells,and the context of the composition or deposition of various specificlayers in embodiments of the product as specified and defined byApplicant is in order.

There are a multitude of properties that should be considered inspecifying and selecting the composition of, inter alia, a specificsemiconductor layer, the back metal layer, the adhesive or bondingmaterial, or the composition of the supporting material for mounting asolar cell thereon. For example, some of the properties that should beconsidered when selecting a particular layer or material are electricalproperties (e.g. conductivity), optical properties (e.g., band gap,absorbance and reflectance), structural properties (e.g., thickness,strength, flexibility, Young's modulus, etc.), chemical properties(e.g., growth rates, the “sticking coefficient” or ability of one layerto adhere to another, stability of dopants and constituent materialswith respect to adjacent layers and subsequent processes, etc.), thermalproperties (e.g., thermal stability under temperature changes,coefficient of thermal expansion), and manufacturability (e.g.,availability of materials, process complexity, process variability andtolerances, reproducibility of results over high volume, reliability andquality control issues).

In view of the trade-offs among these properties, it is not alwaysevident that the selection of a material based on one of itscharacteristic properties is always or typically “the best” or “optimum”from a commercial standpoint or for Applicant's purposes. For example,theoretical studies may suggest the use of a quaternary material with acertain band gap for a particular subcell would be the optimum choicefor that subcell layer based on fundamental semiconductor physics. As anexample, the teachings of academic papers and related proposals for thedesign of very high efficiency (over 40%) solar cells may thereforesuggest that a solar cell designer specify the use of a quaternarymaterial (e.g., InGaAsP) for the active layer of a subcell. A few suchdevices may actually be fabricated by other researchers, efficiencymeasurements made, and the results published as an example of theability of such researchers to advance the progress of science byincreasing the demonstrated efficiency of a compound semiconductormultijunction solar cell. Although such experiments and publications areof “academic” interest, from the practical perspective of the Applicantsin designing a compound semiconductor multijunction solar cell to beproduced in high volume at reasonable cost and subject to manufacturingtolerances and variability inherent in the production processes andsuited for specific applications such as the space environment where theefficiency over the entire operational life is an important goal, suchan “optimum” design from an academic perspective is not necessarily themost desirable design in practice, and the teachings of such studiesmore likely than not point in the wrong direction and lead away from theproper design direction. Stated another way, such references mayactually “teach away” from Applicant's research efforts and the ultimatesolar cell design proposed by the Applicants.

In view of the foregoing, it is further evident that the identificationof one particular constituent element (e.g. indium, or aluminum) in aparticular subcell, or the thickness, band gap, doping, or othercharacteristic of the incorporation of that material in a particularsubcell, is not a single, simple “result effective variable” that oneskilled in the art can simply specify and incrementally adjust to aparticular level and thereby increase the efficiency of a solar cell atthe beginning of life or the end of life, or over a particular time spanof operational use. The efficiency of a solar cell is not a simplelinear algebraic equation as a function of band gap, or the amount ofgallium or aluminum or other element in a particular layer. The growthof each of the epitaxial layers of a solar cell in an MOCVD reactor is anon-equilibrium thermodynamic process with dynamically changing spatialand temporal boundary conditions that is not readily or predictablymodeled. The formulation and solution of the relevant simultaneouspartial differential equations covering such processes are not withinthe ambit of those of ordinary skill in the art in the field of solarcell design.

Another aspect of the disclosure is to match the larger short circuitcurrent of the bottom subcell of the solar cell assembly with two orthree parallel stacks of solar subcells, i.e. a configuration in whichthe value of the short circuit current of the bottom subcell is at leasttwice, or at least three times, that of the solar subcells in eachparallel stack which are connected in series with the bottom subcell.Stated another way, given the choice of the composition of the bottomsubcell, and thereby the short circuit current of the bottom subcell,the upper subcell stack is specified and designated to have a shortcircuit current which is one-third or less or is one-half or less thanthat of the bottom subcell.

Even when it is known that particular variables have an impact onelectrical, optical, chemical, thermal or other characteristics, thenature of the impact often cannot be predicted with much accuracy,particularly when the variables interact in complex ways, leading tounexpected results and unintended consequences. Thus, significant trialand error, which may include the fabrication and evaluative testing ofmany prototype devices, often over a period of time of months if notyears, is required to determine whether a proposed structure with layersof particular compositions, actually will operate as intended, in agiven environment over the operational life, let alone whether it can befabricated in a reproducible high volume manner within the manufacturingtolerances and variability inherent in the production process, andnecessary for the design of a commercially viable device.

Furthermore, as in the case here, where multiple variables interact inunpredictable ways, the proper choice of the combination of variablescan produce new and “unexpected results”, and constitute an “inventivestep” in designing and specifying a solar cell to operate in apredetermined environment (such as space), not only at the beginning oflife, but over the entire defined operational lifetime.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

One aspect of the present disclosure relates to the use and amount ofaluminum in the active layers of the upper subcells in a multijunctionsolar cell (i.e. the subcells that are closest to the primary lightsource). The effects of increasing amounts of aluminum as a constituentelement in an active layer of a subcell affects the photovoltaic deviceperformance. One measure of the “quality” or “goodness” of a solar cellsubcell or junction is the difference between the band gap of thesemiconductor material in that subcell or junction and the V_(oc), oropen circuit voltage, of that same junction. The smaller the difference,the higher the V_(oc) of the solar cell junction relative to the bandgap, and the better the performance of the device. V_(oc) is verysensitive to semiconductor material quality, so the smaller theE_(g)/q−V_(oc) of a device, the higher the quality of the material inthat device. There is a theoretical limit to this difference, known asthe Shockley-Queisser limit. That is the best voltage that a solar celljunction can produce under a given concentration of light at a giventemperature.

The experimental data obtained for single junction (Al)GaInP solar cellsindicates that increasing the Al content of the junction leads to alarger E_(g)/q−V_(oc) difference, indicating that the material qualityof the junction decreases with increasing Al content. FIG. 1 shows thiseffect. The three compositions cited in the Figure are all latticematched to GaAs, but have differing Al composition. As seen by thedifferent compositions represented, with increasing amount of aluminumrepresented by the x-axis, adding more Al to the semiconductorcomposition increases the band gap of the junction, but in so doing alsoincreases E_(g)/q−V_(oc). Hence, we draw the conclusion that adding Alto a semiconductor material degrades that material such that a solarcell device made out of that material does not perform relatively aswell as a junction with less Al.

Thus, contrary to the conventional wisdom as indicated above, thepresent application utilizes a substantial amount of aluminum, i.e.,over 20% aluminum by mole fraction in at least the top subcell, and insome embodiments in one or more of the middle subcells.

Turning to the fabrication of the multijunction solar cell assembly ofthe present disclosure, and in particular a five-junction solar cellassembly, FIG. 2A is a cross-sectional view of a first embodiment of asemiconductor body 500 after several stages of fabrication including thegrowth of certain semiconductor layers on the growth substrate, andformation of grids and contacts on the contact layer of the top side(i.e., the light-facing side) of the semiconductor body.

As shown in the illustrated example of FIG. 2A, the bottom subcell(which we refer to initially as subcell D) includes a substrate 600formed of p-type germanium (“Ge”) in some embodiments, which also servesas a base layer of the subcell (i.e., the p-polarity layer of a“base-emitter” photovoltaic junction formed by adjacent layers ofopposite conductivity type).

In some embodiments, the bottom subcell D is germanium, while in otherembodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, orSiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN,InGaSbBiN or other III-V or II-VI compound semiconductor material.

The bottom subcell D, further includes, for example, a highly dopedn-type Ge emitter layer 601, and an n-type indium gallium arsenide(“InGaAs”) nucleation layer 602. The nucleation layer or buffer 602 isdeposited over the base layer, and the emitter layer 601 is formed inthe substrate by diffusion of atoms from the nucleation layer 602 intothe Ge substrate, thereby forming the n-type Ge layer 601 b.

A highly doped first lateral conduction layer 603 is deposited overlayer 602, and a blocking p-n diode or insulating layer 604 is depositedover the layer 603. A second highly doped lateral conduction layer 605is then deposited over layer 604.

Heavily doped p-type aluminum gallium arsenide (“AlGaAs”) and heavilydoped n-type indium gallium arsenide (“(In)GaAs”) tunneling junctionlayers 606, 607 may be deposited over the second lateral conductionlayer 605 to provide a low resistance pathway between the bottom D andthe middle subcell C.

In some embodiments, distributed Bragg reflector (DBR) layers 608 arethen grown adjacent to and between the tunnel junction 606/607 and thethird solar subcell C₁. The DBR layers 608 are arranged so that lightcan enter and pass through the third solar subcell C₁ and at least aportion of which can be reflected back into the third solar subcell C₁by the DBR layers 608. In the embodiment depicted in FIG. 2A, thedistributed Bragg reflector (DBR) layers 608 are specifically locatedbetween the third solar subcell C and tunnel junction layers 607; inother embodiments, the distributed Bragg reflector tunnel diode layers606/607 may be located between DBR layer 608 and the third subcell C₁.In another embodiment, depicted in FIG. 2C, the tunnel diode layer670/671 are located between the lateral conduction layer 602 d and thefirst alpha layer 606.

For some embodiments, distributed Bragg reflector (DBR) layers 608 canbe composed of a plurality of alternating layers 608 a through 608 z oflattice matched materials with discontinuities in their respectiveindices of refraction. For certain embodiments, the difference inrefractive indices between alternating layers is maximized in order tominimize the number of periods required to achieve a given reflectivity,and the thickness and refractive index of each period determines thestop band and its limiting wavelength.

For some embodiments, distributed Bragg reflector (DBR) layers 608 athrough 608 z includes a first DBR layer composed of a plurality of ptype Al_(x)(In)Ga_(1-x)As layers, and a second DBR layer disposed overthe first DBR layer and composed of a plurality of p typeAl_(y)(In)Ga_(1-y)As layers, where y is greater than x, with 0<x<1,0<y<1.

On top of the DBR layers 608 the subcell C₁ is grown.

In the illustrated example of FIG. 2A, the subcell C₁ includes a highlydoped p-type aluminum gallium arsenide (“Al(In)GaAs”) back surface field(“BSF”) layer 609, a p-type InGaAs base layer 610, a highly doped n-typeindium gallium phosphide (“InGaP2”) emitter layer 611 and a highly dopedn-type indium aluminum phosphide (“AlInP2”) window layer 612. The InGaAsbase layer 610 of the subcell C₁ can include, for example, approximately1.5% In. Other compositions may be used as well. The base layer 610 isformed over the BSF layer 609 after the BSF layer is deposited over theDBR layers 608 a through 608 z.

The window layer 612 is deposited on the emitter layer 611 of thesubcell C₁. The window layer 612 in the subcell C₁ also helps reduce therecombination loss and improves passivation of the cell surface of theunderlying junctions. Before depositing the layers of the subcell B,heavily doped n-type InGaP and p-type Al(In)GaAs (or other suitablecompositions) tunneling junction layers 613, 614 may be deposited overthe subcell C₁.

The middle subcell B₁ includes a highly doped p-type aluminum (indium)gallium arsenide (“Al(In)GaAs”) back surface field (“BSF”) layer 615, ap-type Al(In)GaAs base layer 616, a highly doped n-type indium galliumphosphide (“InGaP2”) or Al(In)GaAs layer 617 and a highly doped n-typeindium gallium aluminum phosphide (“AlGaAlP”) window layer 618. TheInGaP emitter layer 617 of the subcell B₁ can include, for example,approximately 50% In. Other compositions may be used as well.

Before depositing the layers of the top cell A₁, heavily doped n-typeInGaP and p-type Al(In)GaAs tunneling junction layers 619, 620 may bedeposited over the subcell B.

In the illustrated example, the top subcell A₁ includes a highly dopedp-type indium aluminum phosphide (“InAlP”) BSF layer 621, a p-typeInGaAlP base layer 622, a highly doped n-type InGaAlP emitter layer 623and a highly doped n-type InAlP2 window layer 624. The base layer 623 ofthe top subcell A₁ is deposited over the BSF layer 621 after the BSFlayer 621 is formed over the tunneling junction layers 619, 620 of thesubcell B₁. The window layer 624 is deposited over the emitter layer 623of the top subcell A₁ after the emitter layer 623 is formed over thebase layer 622.

In some embodiments, the amount of aluminum in the top subcell A, is 20%or more by mole fraction.

A cap or contact layer 625 may be deposited and patterned into separatecontact regions over the window layer 624 of the top subcell A₁. Afterfurther processing to be described in subsequent Figures, the solar cellassembly 500 can be provided with grid lines, interconnecting bus lines,and contact pads on the top surface. The geometry and number of the gridlines, bus lines and/or contacts may vary in different implementations.

The cap or contact layer 625 serves as an electrical contact from thetop subcell A₁ to metal grid 626. The doped cap or contact layer 625 canbe a semiconductor layer such as, for example, a GaAs or InGaAs layer.

After the cap or contact layer 625 is deposited, the grid lines 626 areformed via evaporation and lithographically patterned and deposited overthe cap or contact layer 625.

A contact pad 627 connected to the grid 626 is formed on one edge of thesubassembly 500 to allow an electrical interconnection to be made, interalia, to an adjacent subassembly.

The subcells A₂, B₂, C₂ of the solar cell subassembly 500 can beconfigured so that the short circuit current densities of the threesubcells A₂, B₂, C₂ have a substantially equal predetermined first value(i.e., J1=J2=J3), and the short circuit current density (J4) of thebottom subcell D₂ is at least twice that of the predetermined firstvalue.

FIG. 2B is a cross-sectional view of a second embodiment of asemiconductor body including a four solar subcells including two latticemismatched subcells with a metamorphic layer between them, after severalstages of fabrication including the growth of certain semiconductorlayers on the growth substrate up to the contact layer, according to thepresent disclosure.

As shown in the previously illustrated example of FIG. 2A, the bottomsubcell (which we refer to initially as subcell D) includes a substrate600 formed of p-type germanium (“Ge”) in some embodiments, which alsoserves as a base layer, and since the layers 601 through 605 aresubstantially the same as described in connection with FIG. 2A, theywill not be described in detail here for brevity.

In the embodiment of FIG. 2B, a first alpha layer 606 a, composed ofn-type (Al)GaIn(As)P, is deposited over the first lateral conductionlayer 605, to a thickness of between 0.25 and 1.0 micron. Such an alphalayer is intended to prevent threading dislocations from propagating,either opposite to the direction of growth into the bottom subcell D, orin the direction of growth into the subcell C, and is more particularlydescribed in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeldet al.).

A metamorphic layer (or graded interlayer) 606 b is deposited over thefirst alpha layer 606 a using a surfactant. Layer 604 is preferably acompositionally step-graded series of InGaAlAs layers, preferably withmonotonically changing lattice constant, so as to achieve a gradualtransition in lattice constant in the semiconductor structure fromsubcell D₁ to subcell C₁ while minimizing threading dislocations fromoccurring. The band gap of layer 606 b is either constant throughout itsthickness, in one embodiment approximately equal to 1.42 to 1.62 eV, orotherwise consistent with a value slightly greater than the band gap ofthe middle subcell C₁, or may vary within the above noted region. Oneembodiment of the graded interlayer may also be expressed as beingcomposed of (Al)In_(x)Ga_(1-x)As, with 0<x<1, and x selected such thatthe band gap of the interlayer is in the range of at approximately 1.42to 1.62 eV or other appropriate band gap.

In the surfactant assisted growth of the metamorphic layer 606 b, asuitable chemical element is introduced into the reactor during thegrowth of layer 606 b to improve the surface characteristics of thelayer. In one embodiment, such element may be a dopant or donor atomsuch as selenium (Se) or tellurium (Te). Small amounts of Se or Te aretherefore incorporated in the metamorphic layer 604, and remain in thefinished solar cell. Although Se or Te are the preferred n-type dopantatoms, other non-isoelectronic surfactants may be used as well.

Surfactant assisted growth results in a much smoother or planarizedsurface. Since the surface topography affects the bulk properties of thesemiconductor material as it grows and the layer becomes thicker, theuse of the surfactants minimizes threading dislocations in the activeregions, and therefore improves overall solar cell efficiency.

As an alternative to the use of non-isoelectronic one may use anisoelectronic surfactant. The term “isoelectronic” refers to surfactantssuch as antimony (Sb) or bismuth (Bi), since such elements have the samenumber of valence electrons as the P atom of InGaP, or the As atom inInGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactantswill not typically be incorporated into the metamorphic layer 606 b.

In one embodiment of the present disclosure, the layer 606 b is composedof a plurality of layers of (Al)InGaAs, with monotonically changinglattice constant, each layer having a band gap, approximately in therange of 1.42 to 1.62 eV. In some embodiments, the band gap is in therange of 1.45 to 1.55 eV. In some embodiments, the band gap is in therange of 1.5 to 1.52 eV.

The advantage of utilizing the embodiment of a constant bandgap materialsuch as InGaAs is that arsenide-based semiconductor material is mucheasier to process in standard commercial MOCVD reactors.

Although the preferred embodiment of the present disclosure utilizes aplurality of layers of (Al)InGaAs for the metamorphic layer 606 b forreasons of manufacturability and radiation transparency, otherembodiments of the present disclosure may utilize different materialsystems to achieve a change in lattice constant from subcell C₁ tosubcell D. Other embodiments of the present disclosure may utilizecontinuously graded, as opposed to step graded, materials. Moregenerally, the graded interlayer may be composed of any of the As, P, N,Sb based III-V compound semiconductors subject to the constraints ofhaving the in-plane lattice parameter greater than or equal to that ofthe third solar cell and less than or equal to that of the fourth solarcell, and having a bandgap energy greater than that of the third solarcell.

A second alpha layer 606 c, composed of n+ type GaInP, is deposited overmetamorphic buffer layer 606 b, to a thickness of between 0.25 and about1.0 micron. Such second alpha layer is intended to prevent threadingdislocations from propagating, either opposite to the direction ofgrowth into the subcell D, or in the direction of growth into thesubcell C₁, and is more particularly described in U.S. PatentApplication Pub. No. 2009/0078309 A₁ (Cornfeld et al.).

A second embodiment of the second solar cell subassembly similar to thatof FIG. 2C is another configuration (not shown) with that themetamorphic buffer layer 604 is disposed above the tunnel diode layers706, 707 and below the DBR layers 708 (not illustrated).

Heavily doped p-type aluminum gallium arsenide (“AlGaAs”) and heavilydoped n-type indium gallium arsenide (“(In)GaAs”) tunneling junctionlayers 607 a, 607 b may be deposited over the alpha layer 606 c toprovide a low resistance pathway between the bottom and middle subcellsD and C₁.

Since the tunnel diode layers 607 a, 607 b, and the subsequently grownlayers 608 through 625 are substantially the same as described inconnection with FIG. 2A, they will not be described here in detail forbrevity.

Turing to FIG. 2C, following the deposition of the semiconductor layers602 through 625, the semiconductor body 501 is partially etched from thebackside (i.e., through the substrate 600) to form a channel 670 thatpartially bisects the wafer or semiconductor body, and several ledges orplatforms are formed on intermediate layers so that electrical contactmay be made thereto, in particular, in one embodiment depicted in thisFigure, ledges 666, and 667.

To this end, the solar cell assembly can include a plurality of openingsin the semiconductor body, each of the openings extending from a bottomsurface of the semiconductor body to a different respective layer in thesemiconductor body. Such “openings” may include recesses, cavities,holes, gaps, cut-outs, or similar structures, but for simplicity we willsubsequently just use the term “opening” throughout this disclosure. Inother implementations, we can etch through the top or the side of thesubstrate and have some or all the openings come from one or more sides.This approach may be more efficient than etching from the top side as itdoes not shadow the top two or top three solar subcells, and results ina solar epitaxial structure of only a few tens of microns in thickness.

FIG. 2D is a cross-sectional view of the embodiment of FIG. 2B followingthe steps of etching contact ledges on various semiconductor layersaccording to a second implementation in the present disclosure. Inparticular, in addition to the two ledges 667 and 666 depicted in FIG.2C, there is a third ledge 668 in the substrate 600 which is etched toallow electrical contact to be made to the p terminal of subcell D.

As a result of the etching process depicted in FIG. 2C or FIG. 2D, thesemiconductor body 501 is divided into two semiconductor regions, withone depicted on the left hand side of the Figure and one on the righthand side. The bottom surface of a portion of the highly doped secondlateral conduction layer 605 is exposed by the etching process and formsa ledge 667. The blocking p-n diode or insulating layer 604 is dividedinto two portions, with one in each of the respective semiconductorregion, one portion 604 a being on the left and one portion 604 b beingon the right of the Figure. Similarly, the first highly doped lateralconduction layer 603 is divided into two parts, with one in eachrespective semiconductor region, one portion 603 a on the left and oneportion 603 b on the right of the Figure. A ledge 666 is formed on theleft portion 603 a of the first highly doped lateral conduction layer603, and a ledge 669 is formed on the right portion 603 b of the firsthighly doped lateral conduction layer 603.

The buffer layer 602 and the subcell D 600/601 is divided into twosemiconductor regions. One portion 602 a of the buffer layer on the lefthand side of the Figure and one portion 602 b of the buffer layer on theright hand side. One portion 600 a/601 a of the solar subcell D (whichwe now designate as solar subcell D₁) on the left hand side of theFigure and one portion 600 a/601 a of the solar cell D (which we nowdesignate as solar subcell D₂) the right hand side. A ledge 668 isformed on the left portion 600 a of the subcell D₁.

After the cap or contact layer 625 is deposited, the grid lines 626 areformed via evaporation and lithographically patterned and deposited overthe cap or contact layer 625.

A contact pad 627 connected to the grid 626 is formed on one edge of thesubassembly 500 to allow an electrical interconnection to be made, interalia, to an adjacent subassembly.

As with the first solar cell subassembly 500, the subcells A₂, B₂, C₂ ofthe second solar cell subassembly 700 can be configured so that theshort circuit current densities of the three subcells A₂, B₂, C₂ have asubstantially equal predetermined first value (J1<=J2<=J3), and theshort circuit current density (J4) of the bottom subcell D₂ is at leasttwice that of the predetermined first value.

FIG. 2E is a cross-sectional view of the multijunction solar cellsubassembly 500 of FIG. 2A after additional stages of fabricationincluding the deposition of metal contact pads on the ledges depicted inFIG. 2D.

A metal contact pad 680 is deposited on the surface of the ledge of 667which exposes a portion of the bottom surface of the lateral conductionlayer 605 b. This pad 680 allows electrical contact to be made to thebottom of the stack of subcells A₁ through C₁.

A metal contact pad 681 is deposited on the surface of the ledge of 666which exposes a portion of the bottom surface of the lateral conductionlayer 603 a. This pad 681 allows electrical contact to be made to then-polarity terminal of subcell D₁.

A metal contact pad 682 is deposited on the surface of the ledge of 669which exposes a portion of the bottom surface of the lateral conductionlayer 603 b. This pad 682 allows electrical contact to be made to then-polarity terminal of subcell D₂.

A metal contact pad 683 is deposited on the surface of the ledge of 668which exposes a portion of the surface of the p-polarity region ofsubcell D₁. Alternatively, contact may be made to a part of the backmetal layer 684, which allows electrical contact to be made to thep-terminal of subcell D₁.

For example, as shown in the bottom plan view depicted in FIG. 3 ,conductive (e.g., metal) interconnections 690 (i.e., 690 a and 690 b),and 691 (i.e., 691 a and 691 b) can be made between different layers ofthe solar cell subregions. Similarly, the interconnection 691 connectstogether a contact 683 on the p-region 600 a of the solar subcell D₁ toa contact 682 on the lateral conduction layer 603 b associated with thesolar subcell D₂. Likewise, the interconnection 690 connects together acontact 681 on the lateral conduction layer 603 a associated with thesolar subcell D₁ to a contact 680 on the lateral conduction layer 605 aand 605 b.

As noted above, the solar cell assembly includes a first electricalcontact of a first polarity and a second electrical contact of a secondpolarity. In some embodiments, the first electrical contact 695 isconnected to the metal contact 627 on the solar cell subassembly 500 byan interconnection 694, and the second electrical contact 693 isconnected to the back metal contact of the solar subcell D₂ byinterconnection 692.

As illustrated in FIG. 3 , two or more solar cell subregions 684 and 685can be connected electrically as described above to obtain amultijunction (e.g. a four-, five- or six-junction) solar cell assembly.

Some implementations provide that at least the base of at least one ofthe first, second or third solar subcells has a graded doping, i.e., thelevel of doping varies from one surface to the other throughout thethickness of the base layer. In some embodiments, the gradation indoping is exponential. In some embodiments, the gradation in doping isincremental and monotonic.

In some embodiments, the emitter of at least one of the first, second orthird solar subcells also has a graded doping, i.e., the level of dopingvaries from one surface to the other throughout the thickness of theemitter layer. In some embodiments, the gradation in doping is linear ormonotonically decreasing.

As a specific example, the doping profile of the emitter and base layersmay be illustrated in FIG. 4 , which depicts the amount of doping in theemitter region and the base region of a subcell. N-type dopants includesilicon, selenium, sulfur, germanium or tin. P-type dopants includesilicon, zinc, chromium, or germanium.

In the example of FIG. 4 , in some embodiments, one or more of thesubcells have a base region having a gradation in doping that increasesfrom a value in the range of 1×10¹⁵ to 1×10¹⁸ free carriers per cubiccentimeter adjacent the p-n junction to a value in the range of 1×10¹⁶to 4×10¹⁸ free carriers per cubic centimeter adjacent to the adjoininglayer at the rear of the base, and an emitter region having a gradationin doping that decreases from a value in the range of approximately5×10¹⁸ to 1×10¹⁷ free carriers per cubic centimeter in the regionimmediately adjacent the adjoining layer to a value in the range of5×10¹⁵ to 1×10¹⁸ free carriers per cubic centimeter in the regionadjacent to the p-n junction.

FIG. 5 is a schematic diagram of the five junction solar cell assemblyof FIG. 2E that includes two solar cell semiconductor regions, each ofwhich includes four subcells. The bottom (i.e., fourth) subcell D₁ ofthe left region 684 is connected in a series electrical circuit with thebottom (i.e., fourth) subcell D₂ of the right region 685. On the otherhand, the upper and middle subcells are connected in parallel with oneanother (i.e., subcells A₁, B₁, C₁ are connected in parallel withsubcells A₂, B₂, C₂).

In some implementations of a five-junction solar cell assembly, such asin the example of FIG. 5 , the short circuit density (J_(sc)) of theupper first subcells (A₁ and A₂) and the middle subcells (B₁, B₂, C₁,C₂) is about 11 mA/cm², and the short circuit current density (J_(sc))of the bottom subcells (D₁ and D₂) is about 22 mA/cm² or greater. Otherimplementations may have different values.

The present disclosure like that of the parallel applications, U.S.patent application Ser. Nos. 14/828,206 and 15/213,594, provides amultijunction solar cell that follows a design rule that one shouldincorporate as many high band gap subcells as possible to achieve thegoal to increase the efficiency at high temperature EOL. For example,high band gap subcells may retain a greater percentage of cell voltageas temperature increases, thereby offering lower power loss astemperature increases. As a result, both high temperaturebeginning-of-life (HT-BOL) and HT-EOL performance of the exemplarymultijunction solar cell, according to the present disclosure, may beexpected to be greater than traditional cells.

The open circuit voltage (V_(oc)) of a compound semiconductor subcellloses approximately 2 mV per degree C. as the temperature rises, so thedesign rule taught by the present disclosure takes advantage of the factthat a higher band gap (and therefore higher voltage) subcell loses alower percentage of its V_(oc) with temperature. For example, a subcellthat produces a 1.50 volts at 28° C. produces 1.50−42*(0.0023)=1.403volts at 70° C. which is a 6.4% voltage loss, A cell that produces 0.25volts at 28° C. produces 0.25−42*(0.0018)=0.174 volts at 70° which is a30.2% voltage loss.

For example, the cell efficiency (%) measured at room temperature (RT)28° C. and high temperature (HT) 70° C., at beginning of life (BOL) andend of life (EOL), for a standard three junction commercial solar cell(e.g. a SolAero Technologies Corp. Model ZTJ), such as depicted in FIG.2 of U.S. patent application Ser. No. 14/828,206, is as follows:

Condition Efficiency BOL 28° C. 29.1% BOL 70° C. 26.4% EOL 70° C. 23.4%After 5E14 e/cm² radiation EOL 70° C. 22.0% After 1E15 e/cm² radiation

For the 5J solar cell assembly (comprising two interconnectedfour-junction subassemblies) described in the present disclosure, thecorresponding data is as follows:

Condition Efficiency BOL 28° C. 30.6% BOL 70° C. 27.8% EOL 70° C. 26.6%After 5E14 e/cm² radiation EOL 70° C. 26.1% After 1E15 e/cm² radiation

The new solar cell of the present disclosure has a slightly higher cellefficiency than the standard commercial solar cell (ZTJ) at BOL at 70°C. However, more importantly, the solar cell described in the presentdisclosure exhibits substantially improved cell efficiency (%) over thestandard commercial solar cell (ZTJ) at 1 MeV electron equivalentfluence of 5×10¹⁴ e/cm², and dramatically improved cell efficiency (%)over the standard commercial solar cell (ZTJ) at 1 MeV electronequivalent fluence of 1×10¹⁵ e/cm².

The selected radiation exposure levels noted above are meant to simulatethe environmental conditions of typical satellites in earth orbit. A lowearth orbit (LEO) satellite will typically experience radiationequivalent to 5×10¹⁴ e/cm² over a five year lifetime. A geosynchronousearth orbit (GEO) satellite will typically experience radiation in therange of 5×10¹⁴ e/cm² to 1×10 e/cm² over a fifteen year lifetime.

The wide range of electron and proton energies present in the spaceenvironment necessitates a method of describing the effects of varioustypes of radiation in terms of a radiation environment which can beproduced under laboratory conditions. The methods for estimating solarcell degradation in space are based on the techniques described by Brownet al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of theTelstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963]and Tada [Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G.Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication82-69, 1982]. In summary, the omnidirectional space radiation isconverted to a damage equivalent unidirectional fluence at a normalisedenergy and in terms of a specific radiation particle. This equivalentfluence will produce the same damage as that produced by omnidirectionalspace radiation considered when the relative damage coefficient (RDC) isproperly defined to allow the conversion. The relative damagecoefficients (RDCs) of a particular solar cell structure are measured apriori under many energy and fluence levels. When the equivalent fluenceis determined for a given space environment, the parameter degradationcan be evaluated in the laboratory by irradiating the solar cell withthe calculated fluence level of unidirectional normally incident flux.The equivalent fluence is normally expressed in terms of 1 MeV electronsor 10 MeV protons.

The software package Spenvis (www.spenvis.oma.be) is used to calculatethe specific electron and proton fluence that a solar cell is exposed toduring a specific satellite mission as defined by the duration,altitude, azimuth, etc. Spenvis employs the EQFLUX program, developed bythe Jet Propulsion Laboratory (JPL) to calculate 1 MeV and 10 MeV damageequivalent electron and proton fluences, respectively, for exposure tothe fluences predicted by the trapped radiation and solar proton modelsfor a specified mission environment duration. The conversion to damageequivalent fluences is based on the relative damage coefficientsdetermined for multijunction cells [Marvin, D. C., Assessment ofMultijunction Solar Cell Performance in Radiation Environments,Aerospace Report No. TOR-2000 (1210)-1, 2000]. A widely accepted totalmission equivalent fluence for a geosynchronous satellite mission of 15year duration is 1 MeV 1×10¹⁵ electrons/cm².

The exemplary solar cell described herein may require the use ofaluminum in the semiconductor composition of each of the top twosubcells. Aluminum incorporation is widely known in the III-V compoundsemiconductor industry to degrade BOL subcell performance due to deeplevel donor defects, higher doping compensation, shorter minoritycarrier lifetimes, and lower cell voltage and an increased BOLE_(g)/q−V_(oc) metric. However, consideration of the EOL E_(g)/q−V_(oc)metric, or more generally, total power output over a definedpredetermined operational life, in the present disclosure presents adifferent approach. In short, increased BOL E_(g)/q−V_(oc) may be themost problematic shortcoming of aluminum containing subcells; the otherlimitations can be mitigated by modifying the doping schedule orthinning base thicknesses.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofstructures or constructions differing from the types of structures orconstructions described above.

Although described embodiments of the present disclosure utilizes avertical tandem stack of four subcells, various aspects and features ofthe present disclosure can apply to tandem stacks with fewer or greaternumber of subcells, i.e. two junction cells, three junction cells, fivejunction cells, etc.

In addition, although the disclosed embodiments are configured with topand bottom electrical contacts, the subcells may alternatively becontacted by means of metal contacts to laterally conductivesemiconductor layers between the subcells. Such arrangements may be usedto form 3-terminal, 4-terminal, and in general, n-terminal devices. Thesubcells can be interconnected in circuits using these additionalterminals such that most of the available photogenerated current densityin each subcell can be used effectively, leading to high efficiency forthe multijunction cell, notwithstanding that the photogenerated currentdensities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure mayutilize an arrangement of one or more, or all, homojunction cells orsubcells, i.e., a cell or subcell in which the p-n junction is formedbetween a p-type semiconductor and an n-type semiconductor both of whichhave the same chemical composition and the same band gap, differing onlyin the dopant species and types, and one or more heterojunction cells orsubcells. Subcell A, with p-type and n+ type InGaAlP is one example of ahomojunction subcell.

In some cells, a thin so-called “intrinsic layer” may be placed betweenthe emitter layer and base layer, with the same or different compositionfrom either the emitter or the base layer. The intrinsic layer mayfunction to suppress minority-carrier recombination in the space-chargeregion. Similarly, either the base layer or the emitter layer may alsobe intrinsic or not-intentionally-doped (“NID”) over part or all of itsthickness.

The composition of the window or BSF layers may utilize othersemiconductor compounds, subject to lattice constant and band gaprequirements, and may include AlInP, AlAs, AlP, GaP, InP, GaSb, AlSb,InAs, InSb, ZnSe, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs,GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb,GaInSb, AlGaInSb, AIN, GaN, InN, GaInN, AlGaInN, GaInNAs, GaInNAsSb,GaNAsSb, GaNAsInBi, GaNAsSbBi, GaNAsInBiSb, AlGaInNAs, ZnSSe, CdSSe,SiGe, SiGeSn, and similar materials, and still fall within the spirit ofthe present invention.

While the solar cell described in the present disclosure has beenillustrated and described as embodied in a conventional multijunctionsolar cell, it is not intended to be limited to the details shown, sinceit is also applicable to inverted metamorphic solar cells, and variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Thus, while the description of the semiconductor device described in thepresent disclosure has focused primarily on solar cells or photovoltaicdevices, persons skilled in the art know that other optoelectronicdevices, such as thermophotovoltaic (TPV) cells, photodetectors andlight-emitting diodes (LEDS), are very similar in structure, physics,and materials to photovoltaic devices with some minor variations indoping and the minority carrier lifetime. For example, photodetectorscan be the same materials and structures as the photovoltaic devicesdescribed above, but perhaps more lightly-doped for sensitivity ratherthan power production. On the other hand LEDs can also be made withsimilar structures and materials, but perhaps more heavily-doped toshorten recombination time, thus radiative lifetime to produce lightinstead of power. Therefore, this invention also applies tophotodetectors and LEDs with structures, compositions of matter,articles of manufacture, and improvements as described above forphotovoltaic cells.

Without further analysis, from the foregoing others can, by applyingcurrent knowledge, readily adapt the present invention for variousapplications. Such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

The invention claimed is:
 1. A multijunction solar cell comprising: (a)a single monolithic semiconductor substrate having a top side and abottom side opposite to the top side; (b) a bottom solar subcell regionin the semiconductor substrate including a nucleation layer deposited onthe top side of the semiconductor substrate, the bottom solar subcellregion having an emitter region and a base region disposed below theemitter region, the emitter region of the bottom solar subcell being ona portion of the semiconductor substrate and comprising atoms diffusedfrom the nucleation layer into the semiconductor substrate from the topside thereof; (c) an epitaxial semiconductor structure including asequence of semiconductor epitaxial layers on the nucleation layer, thesemiconductor structure including a first and second adjacent andparallel semiconductor regions with respect to incoming illumination,each of the first and second regions including an upper first solarsubcell and a respective portion of the bottom solar subcell region; and(d) an opening in the semiconductor substrate extending from the bottomside of the semiconductor substrate through the bottom solar subcellregion and through a portion of the sequence of semiconductor layers andterminating at one or more of the semiconductor epitaxial layers grownon the nucleation layer, the multijunction solar cell being configuredfor illumination through a side of the multijunction solar cell that isopposite a side in which the opening is present, the opening in thesemiconductor substrate being a channel that divides the semiconductorsubstrate and a portion of the epitaxial semiconductor structure intothe first and second parallel semiconductor regions, wherein the bottomsolar subcell region is divided by the channel to form discrete,spaced-apart first and second bottom solar subcells (D₁ and D₂respectively) in the respective first and second semiconductor regions,and wherein the opening defines multiple ledges located, respectively,at different distances from the bottom side of the semiconductorsubstrate, the multijunction solar cell further comprising at least oneelectrical contact pad on each of the ledges, and electricalinterconnections connecting together respective ones of the electricalcontact pads on different ones of the ledges.
 2. A multijunction solarcell as defined in claim 1, wherein one of the electricalinterconnections includes a discrete electrical interconnect connectingthe first bottom solar subcell of the first semiconductor region in aseries electrical circuit with the second bottom solar subcell of thesecond semiconductor region so that at least a three junction solar cellis formed by electrically interconnected upper solar subcells, the firstbottom solar subcell, and the second bottom solar subcell.
 3. Amultijunction solar cell as defined in claim 1, further comprising: (a)a first highly doped lateral conduction layer disposed adjacent to andabove the nucleation layer to allow discrete electrical contacts to bemade to the emitter region of the first bottom solar subcell (D₁) andthe emitter region of the second bottom solar subcell (D₂); (b) ablocking p-n diode or insulating layer disposed adjacent to and abovethe first highly doped lateral conduction layer; and (c) a second highlydoped lateral conduction layer disposed adjacent to and above theblocking p-n diode or insulating layer, the second highly doped lateralconduction layer having an upper surface and a bottom surface.
 4. Amultijunction solar cell as defined in claim 3, wherein a portion of thehighly doped lateral conduction layer disposed adjacent to and above thefirst bottom solar subcell (D₁) in the first semiconductor region formsa terminal of first polarity of the first bottom solar subcell, andwherein a portion of the highly doped lateral conduction layer disposedadjacent to and above the second bottom solar subcell (D₂) in the secondregion forms a terminal of first polarity of the second bottom solarsubcell (D₂), the multijunction solar cell further comprising a firstdiscrete metal interconnect extending across the channel andelectrically connecting a terminal of second polarity of the firstbottom solar subcell (D₁) with a terminal of first polarity of thesecond bottom solar subcell (D₂), thereby forming a series of electricalconnection between the first bottom solar subcell (D₁) and the secondbottom solar subcell (D₂).
 5. A multijunction solar cell as defined inclaim 3, wherein the blocking p-n diode or insulating layer disposedadjacent to and above the first highly doped lateral conduction layerprevents current from flowing directly from the second semiconductorregion into the second bottom solar subcell (D₂).
 6. A multijunctionsolar cell as defined in claim 5, wherein the current generated in thesecond semiconductor region is entirely transferred to the first bottomsolar subcell (D₁) in the first semiconductor region, and the sequenceof semiconductor layers in the second semiconductor region iselectrically isolated from the second bottom solar subcell (D₂) in thesecond semiconductor region by the p-n diode disposed above the highlydoped lateral conduction layer so that current does not flow into thesecond bottom solar subcell (D₂) in the second semiconductor regionthrough the semiconductor layers in the second semiconductor region. 7.A multijunction solar cell as defined in claim 1, wherein the at leastone electrical contact pad includes first and third metal contact pads,the first metal contact pad being disposed on a first one of the ledgesadjacent a top surface of the second bottom solar subcell (D₂) in thesecond semiconductor region, the multijunction solar cell including asecond metal contact pad on a bottom surface of the second bottom solarsubcell (D₂) in the second semiconductor region, and wherein the thirdmetal contact pad is disposed on a second one of the ledges on the baseregion of the first bottom solar subcell (D₁) of the first semiconductorregion and is electrically coupled with the first metal contact pad. 8.A multijunction solar cell as defined in claim 7, wherein themultijunction solar cell includes a terminal of first polarity connectedto the upper first solar subcell in the first and second semiconductorregions, and a terminal of second polarity connected to the second metalcontact of the second bottom solar subcell.
 9. A multijunction solarcell as defined in claim 3, wherein the channel comprises: (i) a firstportion that extends from the bottom side of the semiconductor substratethrough the bottom solar subcell region, the first lateral conductionlayer, and the blocking p-n diode or insulating layer, and terminates atthe bottom surface of the second highly doped lateral conduction layer,thereby permitting an electrical contact to be made to the second highlydoped lateral conduction layer from the bottom side of the semiconductorsubstrate, and wherein the first portion of the channel separates afirst portion of the first lateral conduction layer disposed in thefirst semiconductor region from a second portion of the first lateralconduction layer disposed in the second semiconductor region; (ii) asecond portion that extends from the bottom side of the semiconductorsubstrate through the bottom solar subcell region, and terminates at thebottom surface of the first highly doped lateral conduction layer,thereby permitting a first electrical contact to be made to the firstportion of the first lateral conduction layer, and a second electricalcontact to be made to the second portion of the first lateral conductionlayer from the bottom side of the semiconductor substrate, the secondportion of the channel being wider than the first portion of thechannel, and the first portion of the channel being laterally encircledby the second portion; and (iii) a third portion that extends from thebottom side of the semiconductor substrate through a portion of the baseregion of the bottom solar subcell, thereby permitting an electricalcontact to be made to the base region of the bottom solar subcell fromthe bottom side of the semiconductor substrate, the third portion of thechannel being wider than the second portion of the channel and having anedge on one side that is aligned with an edge of the second portion ofthe channel adjacent to the second semiconductor region.
 10. Amultijunction solar cell as defined in claim 9, wherein one of theelectrical interconnections includes a discrete electrical interconnectextending in free space in the opening connecting the first portion ofthe first lateral conduction layer with the second lateral conductionlayer so as to make a series electrical connection between the uppersolar subcell, and the first bottom solar subcell in the firstsemiconductor region.
 11. A multijunction solar cell as defined in claim9, wherein a succession of the first, second and third portions of thechannel forms ledges on the first and second lateral conduction layersso that electrical contact may be made to the ledges on such respectivelayers.
 12. A multijunction solar cell as defined in claim 11, whereinthe opening in the semiconductor substrate terminates at the firsthighly doped lateral conduction layer.
 13. A multijunction solar cell asdefined in claim 1, wherein the semiconductor structure includes threesolar subcells in addition to the bottom solar subcell, the three solarsubcells including an upper first solar subcell, a second solar subcell,and a third solar subcell, and wherein the bottom solar subcell has aband gap of approximately 0.67 eV, the third solar subcell above thebottom solar subcell has a band gap in the range of approximately 1.41eV and 1.31 eV, the second solar subcell above the third solar subcellhas a band gap in the range of approximately 1.65 to 1.8 eV, and theupper first solar subcell has a band gap in the range of 2.0 to 2.20 eV.14. A multijunction solar cell as defined in claim 13, wherein the thirdsolar subcell has a band gap of approximately 1.37 eV, the second solarsubcell has a band gap in the range of approximately 1.73 eV and theupper first solar subcell has a band gap of approximately 2.10 eV.
 15. Amultijunction solar cell as defined in claim 13, wherein: the upperfirst subcell is composed of indium gallium aluminum phosphide; thesecond solar subcell includes an emitter layer composed of indiumgallium phosphide or aluminum gallium arsenide, and a base layercomposed of aluminum gallium arsenide or indium gallium arsenidephosphide; and the third solar subcell is composed of indium galliumarsenide; the multijunction solar cell further comprising a gradedinterlayer disposed between the third solar subcell and the bottom solarsubcell, wherein the graded interlayer is composed of(Al)In_(x)Ga_(1-x)As or In_(x)Ga_(1-x)P with 0<x<1, and (Al) designatesthat aluminum is an optional constituent, and the band gap of the gradedinterlayer is in the range of 1.41 eV to 1.6 eV throughout itsthickness.
 16. A multijunction solar cell as defined in claim 13,further comprising: a distributed Bragg reflector (DBR) layer adjacentto the third solar subcell and arranged so that light can enter and passthrough the third solar subcell and at least a portion of which can bereflected back into the third solar subcell by the DBR layer, and theDBR layer is composed of a plurality of alternating layers of latticematched materials with discontinuities in their respective indices ofrefraction and the difference in refractive indices between alternatinglayers is maximized in order to minimize the number of periods requiredto achieve a given reflectivity, and the thickness and refractive indexof each period determines the stop band and its limiting wavelength, andwherein the DBR layer includes a first DBR layer composed of a pluralityof p type Al_(x)Ga_(1-x) (In)As layers, and a second DBR layer disposedover the first DBR layer and composed of a plurality of n typeAl_(y)Ga_(1-y)(In)As layers, where 0<x<1, 0<y<1, and y is greater thanx, and (In) designates that indium is an optional constituent.